1. Field of the Invention
The present invention relates to an oscillator with a buffer circuit, for use in a portable telephone or the like.
2. Description of the Related Art
FIG. 5 shows the construction of a conventional oscillator with a buffer circuit. An oscillation circuit 41 is of a common-collector connection type and comprises: an oscillation transistor 41a; a grounding capacitor 41b for connecting the collector of the oscillation transistor 41a to the ground so as to pass high frequencies; a feedback capacitor 41c connected between the base and the emitter of the oscillation transistor 41a; a feedback capacitor 41d connected between the emitter the oscillation transistor 41a and the ground; an inductance device 41e connected between the base of the oscillation transistor 41a and the ground; and a varactor diode 41f. The inductance device 41e takes the form of a strip line.
A buffer circuit 42 has an amplification transistor 42a and is an amplification circuit of a common-emitter connection type. The emitter of the amplification transistor 42a has a D.C. ground connection via an emitter bias resistor 42b and, further, connected to the ground via a bypass capacitor 42c so as to pass high frequencies.
The emitter of the oscillation transistor 41a and the collector of the amplification transistor 42a are connected via a choke inductor 43. A common collector current is passed to the oscillation transistor 41a and the amplification transistor 42a by a power supply voltage Vb applied to the collector of the oscillation transistor 41a. 
A control voltage Vc is applied to the cathode of the varactor diode 41f via a choke inductor 44, the oscillation circuit 41 oscillates at a frequency determined by the voltage value, and the oscillation signal is outputted from the emitter of the oscillation transistor 41a and is supplied to the base of the amplification transistor 42a via a coupling capacitor 45. The amplified oscillation signal is outputted from the collector of the amplification transistor 42a to a load via a d.c. blocking capacitor 46.
In the conventional oscillator with the buffer circuit, since the emitter of the oscillation transistor 41a and the collector of the amplification transistor 42a are connected to each other via the choke inductor 43 and the potential at the emitter and that at the collector are equal to each other. The power supply voltage Vb is therefore applied to the collector of the amplification transistor 42a with little attenuation of ripples of the power supply voltage Vb, and the ripple component is multiplexed on the oscillation signal amplified by the buffer circuit 42. Consequently, the C/N ratio of the oscillation signal to be outputted deteriorates.
Since the phase of an amplified oscillation signal appearing at the collector of the amplification transistor 42a and that of an oscillation signal to be applied from the emitter of the oscillation transistor 41a to the collector of the amplification transistor 42a via the choke inductor 43 are opposite to each other, the level of the oscillation signal outputted from the collector of the amplification transistor 42a decreases.
Further, since the emitter of the oscillation transistor 41a is connected to the load via the choke inductor 43 and the d.c. blocking capacitor 46, the load and the choke inductor 43 are arranged in series and interposed as an equivalent feedback capacitance between the emitter of the oscillation transistor 41a and the ground. The equivalent feedback capacitance changes according to the frequency characteristic of the reactance component of the load and a problem such that the oscillation frequency fluctuates occurs.
It is an object of the invention to provide an oscillator with a buffer circuit in which deterioration in the C/N ratio due to ripples of the power supply voltage is lessened, fluctuations in the oscillation frequency caused by a change in the reactance of a load is suppressed and, further, an oscillation signal is sufficiently amplified by a buffer circuit.
In order to achieve the subject, according to the invention, there is provided an oscillator with a buffer circuit, comprising: an oscillation circuit having an oscillation transistor; and a buffer circuit which has an amplification transistor and amplifies an oscillation signal outputted from the oscillation circuit by the amplification transistor, wherein a power supply voltage is applied to the collector of either the oscillation transistor or the amplification transistor, the emitter of the other transistor has a D.C. ground connection, and the emitter of the transistor to which the power supply voltage is applied is connected to the collector of the other transistor via a resistor.
In the oscillator with a buffer circuit according to the invention, the power supply voltage is applied to the collector of the oscillation transistor, the oscillation circuit is of a common-collector connection type, the buffer circuit is an amplification circuit of a common-base connection type, and an oscillation signal is supplied from the emitter of the oscillation transistor to the emitter of the amplification transistor.
In the oscillator with a buffer circuit according to the invention, the power supply voltage is applied to the collector of the oscillation transistor, the oscillation circuit is an oscillation circuit of a common-base connection type, an amplification circuit of a common-emitter connection type is used as the buffer circuit, and an oscillation signal is supplied from the emitter of the oscillation transistor to the base of the amplification transistor.
In the oscillator with a buffer circuit according to the invention, the base of the oscillation transistor and the base of the amplification transistor are connected via a bias resistor, a base bias voltage is applied to the oscillation transistor, and the emitter of the amplification transistor has a D.C. ground connection.
In the oscillator with a buffer circuit according to the invention, the base of the oscillation transistor and the base of the amplification transistor are connected via a bias resistor, a base bias voltage is applied to the oscillation transistor, and the emitter of the amplification transistor is directly connected to the ground.
In the oscillator with a buffer circuit according to the invention, the power supply voltage is applied to the collector of the amplification transistor and an -amplifier of a common-emitter connection type is used as the buffer circuit.
In the oscillator with a buffer circuit according to the invention, the oscillation circuit is of a common-collector connection type and an oscillation signal is supplied from the emitter of the oscillation transistor to the base of the amplification transistor.
In the oscillator with a buffer circuit according to the invention, the base of the amplification transistor and the base of the oscillation transistor are connected via a bias resistor, a base bias voltage of the amplification transistor is applied, and the emitter of the oscillation transistor has a D.C. ground connection.
In the oscillator with a buffer circuit according to the invention, the base of the amplification transistor and the base of the oscillation transistor are connected via a bias resistor, a base bias voltage of the amplification transistor is applied, and the emitter of the oscillation transistor is directly connected to the ground.